In some wireless communications systems, it may be desired to broadcast digital video or other information signals, e.g., radio show broadcasts, to mobile users via a downlink. As mobile nodes move throughout the system, it is desirable that the user of the mobile node be able to receive and decode a continuous or nearly continuous program signal, e.g., a television show to be decoded and displayed in real time. One approach employed is for adjacent base stations in the system to simultaneously broadcast the same signals, with timing synchronization between the base stations' transmitters being controlled to the symbol level. The same information is transmitted at the same time on the same tones by different adjacent base stations. This approach has the disadvantage of requiring a high level of symbol transmission timing synchronization between base stations so that signals received from different base stations do not differ in time by more than a small portion of a symbol transmission time period.
FIG. 1 is a drawing 100 showing an example where two adjacent base stations (BS A 102, BS B 104) transmit the same signal at the same time using a single carrier frequency (C) and the same sub-carrier frequencies, e.g., tones, conveying modulation symbols conveying encoded digital broadcast information bits. In FIG. 1, a mobile node 106 is located equidistant from BS A 102 and BS B 104. With respect to the MN 106, signal A (SA) 110 from BS A 102 arrives at the same time as signal B (SB) 112 from BS B 104, as illustrated by SA 110 and SB 112 being aligned in FIG. 1 with respect to line 108, where line 108 represents the equidistance point between the two base stations (102, 104). Distance is used herein to indicate a travel time of a signal from one point to another, e.g., from a base station to a mobile node. Circumstances in the environment, e.g., reflecting objects, could make a signal travel time different from the straight-line distance between points. Distance is used for convenience of illustration. For symbol timing synchronization to be maintained between the signals received from the different base stations, the BSs (102, 104) need to be tightly synchronized and the synchronization level maintained between the base stations. SA 110 and SB 112 each include payload information (114, 116), respectively, e.g., a modulation symbol value portion, and a cyclic prefix portion (CP), (118, 120), respectively, used for synchronization. The signals (SA 110 and SB 112) combine over the airlink, and the combined signal is received and decoded by the MN 106 recovering the information bits.
FIG. 2 is a drawing 200 illustrating that when the MN 106 is not equidistance from the two BSs (102,104), the received signals (SA 110′, SB 112′) will tend to lose synchronization relative to one another, the amount of synchronization loss being a function of signal path distance differences between the MN and each BS. SB 112′ is delayed with respect to signal SA 110′ from the MNs 106 perspective. The MN 106 may be able to recover received signals in which there is at least some overlap between the cyclic prefixes, e.g., in cases where the signal delay difference between two signals does not exceed the duration of the cyclic prefix. SA 110′ includes payload portion 114′ and CP portion 118′; SB 112′ includes payload portion 116′ and CP portion 120′. Drawing 200 illustrates partial overlap between CP 118′ and CP 120′ from the perspective of the MN 106 receiving both signals 110′ and 112′.
FIG. 3 is a drawing 300 illustrating an example where the MN 106 is located such that the cyclic prefix 118″ from SA 110″ does not overlap with the cyclic prefix 120″ from SB 112″, so that SA 110″ interferes with SB 112″ and vice versa. The MN 106 would typically be unable to recover and decode such a broadcast signal due to the degradation in signal quality resulting from this interference. FIG. 4, shows one known approach used to remedy this problem. The length of the cyclic prefix is increased, thus allowing a larger overlap region. However, the cyclic prefix represents signaling overhead, thus any increase in cyclic prefix length corresponds to a decrease in information bit throughput in the system.
Compare MN 106 received signal timing of FIG. 3 and FIG. 4. In FIG. 3 SA 110″ includes payload information 114″ and CP 118″, and SB 112″ includes payload information 116″ and CP 120″. In FIG. 4 SA 110′″ includes payload information 114′″ and CP 118′″, and SB 112′″ includes payload information 116′″ and CP 120′″. Note that CPs (118′″ and 120′″) are longer in duration than CPs (118″ and 120″); however, payload information portions (114′″ and 116′″) are shorter in duration than payload information portions (114″ and 116″). The increase in CP duration represented by FIG. 4 has resulted in an overlap between CPs (118′″ and 120′″) facilitating the possibility of successful recovery of the payload information; however, this comes at a cost of decrease in payload.
FIG. 5 is a drawing 500 illustrating that exemplary OFDM downlink tones have a tone interspacing. N exemplary downlink tones (tone 1 502, tone 2, 504, tone 3 506, tone 4 508, . . . , tone N-1 510, tone N 512) are shown with tone interspacing delta f 514. One approach that can be used to compensate for lost capacity due to larger cyclic prefix length, is to make the tone spacing smaller than might otherwise have been done, thus fitting more tones into a given frequency band. This approach can pack more information bits into the same frequency capacity assuming that all the tones can still be received reliably. This approach of decreasing tone spacing is bad for mobility, and particularly high velocity mobility users, e.g., a user traveling in a car, bus, or train, since motion can distort the perceived frequency of the signals making it difficult to reliably decode closely spaced tones.
In view of the above discussion, it should be appreciated that known approaches of simulcast broadcasting, e.g., digital video broadcasting, often include some or all of the following undesirable effects: (i) symbol transmission timing synchronization needs to be maintained to within a high degree between different base stations, e.g., within the duration of a cyclic prefix or less; (ii) the cyclic prefix needs to be relatively lengthy resulting in an undesirable amount of overhead, and (iii) the use of narrow tone spacing tends to interfere with reception and processing by mobile devices which can result in insufficient support for mobility. In addition, when channel conditions are not uniform quality may tend to be degraded.
In view of the above discussed problems, there is a need for new methods and apparatus to facilitate downlink broadcasting, e.g., downlink digital video broadcasting, in an OFDM wireless communications system which reduce and/or overcome one or more of the above discussed problems.